This research proposal is based on the hypothesis that some molecules present on the surface of developing cells play critical roles in cellular differentiation and that these molecules can be recognized as stage-specific cell-surface antigens. Stage-specific antigens are restricted to a particular phase of differentiation and presumably perform a function specific to that phase. Germ cells, from their origin as primordial germ cells to formation of spermatozoa, and teratocarcinomas of several different origins and developmental potentials will be used to raise antisera and for definition and quantitation of the antigens in a quantitative radioimmune-binding assay. Teratomas will be used as a readily available model system for differentiation antigens of germ cells and may provide information relating differentiation and tumor-cell antigenicity. The stage-specific antigens will also be used as markers for cells in different stages of differentiation and for separation of the cells on the basis of their antigenicity. The cells will then be available for functional analysis of developmental surface molecules and of cell interactions and tissue organization in development. Some of the differentiation antigens will be biochemically purified and their antigenic properties will be used as markers during the purification process.